1. Warning message displays if the current selection contains elements that cannot be included in an assembly.
The following elements cannot be included in an assembly:
  • Annotations and detail items
  • Assemblies and elements that are already part of another assembly
  • Complex structures (trusses, beam systems, curtain systems, curtain walls, stacked walls)
  • Elements in different design options
  • Groups
  • Imports
  • Images
  • Links or elements in a link
  • Masses
  • MEP-specific elements (ducts, pipes, conduits, cable trays and fittings, HVAC zones)
  • Model lines
  • Rooms
  • Structural loads, load cases, and internal loads

See the rest at:
Assembly Usage Tips – WikiHelp

“information can be provided to the material by linking to a seperately defined property set (by Property Set), or can be input directly into the material (Independent). This allows several materials to share a single set of properties and changes to that property set to propagate to all those materials.

Read the rest of this informative post at:
Revit OpEd: Revit 2012 Materials and Property Sets

Creating an Appearance Library – WikiHelp

Also from the WikiHelp on this subject:
Each material can have two types of properties referred to as aspects: appearance and structural. You can store appearance and structural property sets in their own library. Appearance property libraries adhere to the Autodesk standard Protein format and have the following file extension: .adsklib. 
 Structural property libraries are written in an XML format and have the following file extension: .adstlib. 

When a user appearance property set library is loaded, it is always available in the Materials dialog. Structural user libraries are available per project. They will not load unless you specifically load them into a project. 

You can create new property set libraries in Revit or by creating .adsklib files with an external editing application.
Managing Property Set Libraries – WikiHelp

Release 2011: The Families Guide was not updated for Revit 2011. Please use the 2010 edition of the Families Guide.

Autodesk – Autodesk Revit Architecture Services & Support – Families Guide

Do you get the feeling that perhaps, if Autodesk put it off long enough, they won’t have to update some of this documentation?

With the official Wiki now in operation, and a plethora of blogs, where will Autodesk draw the line with regards to Help documentation?